Stellar orientation detector



June 14, 1960 w GREENLEE ETAL 2,941,081

STELLAR ORIENTATION DETECTOR I Filed Sept. 27, 1950 2 Sheets-Sheet 1 FIG. 2

l3 PHASE V AMPLIFIER SHIFTER COMPUTER 2 l7 2 l9 l6 AMPLIFIER k DIFFERENTIAL s DIFFERENTIAL I sYNcHRo sYNcI-IRo s AMPLIFIER 2 J FIG.3

INVENTORB WILLIAM B. GREENLEE VICTOR A. MILLER ATTORNEY FIG. 4 BY June 14, 1960 w. B. GREENLEE ETAL STELLAR ORIENTATION DETECTOR 2 Sheets-Sheet 2 Filed Sept. 27, 1950 lllli INVENTORS WILLlAM B. GREENLEE BY VICTOR A. MILLER MECH.

DIFF.

FIG.

ATTORNEY United States STELLAR ORIENTATION DETECTOR Filed Sept. 27, 1950, Ser. No. 186,962

17 Claims. (Cl. 250-203) This invention pertains to the detection of light from a star, and particularly to the alignment of an optical axis with the line of sight to a star and the detection of the amount and direction of the deviation of an Optical axis from a line of sight to a star.

This invention contemplates a system for relating the angular position of an optical axis to the line of sight to a star. In the use of light from stars for the automatic navigation of ships or aircraft it has proved possible to use photoelectric means for detecting the presence of a star within the. field of vision of the optical system employed for gathering starlight. However, a serious .problem exists in aligning the optical axis of the telescope or other optical apparatus employed with a line of sight to a star and correcting small angular deviations between the optical axis and the line of sight. In order to determine in what direction a star image is displaced from an optical axis, it has been proposed to modulate the light received through the optical system by means of a totating reticle and cast the modulated light On a photocell. In this connection reference is made to application Serial No. 173,146 entitled Star Sensing System, filed July 11, 1950, in the name of William B. Greenlee and Victor A. Miller. A phase comparison of the modulated signal with a reference signal indicates in what direction the star image is displaced from the optical axis. However, it is desirable for navigation purposes to know not only the direction of such deviation but also, at least roughly, the amount thereof in order that the proper amount of correction in orientation may be applied to the telescope optical axis.

Another problem encountered is that ofsky gradient signal. Though the background sk'y light is of relatively low intensity during the night, its intensity is much greater during daylight hours. Furthermore, except in that part of the sky included in a 30 cone around the line of sight to the sun, the sky light varies as much as 3 percent per degree over the field of view of a telescope of the type which may be used to detect stars, and may be assumed to vary linearly. This variation in sky light may produce a modulated signal on a photocell similar to that caused by a star in the field, thus falsely indicating the presence of a star. The effect of sky light must therefore be reduced to a negligible value if possible and false signals generated because of variations in sky light, i.e., sky gradient, must be eliminated.

It is therefore an object of this invention to provide a star sensing system adapted to yield an electrical signal indicative of the magnitude and direction of the deviation of an optical axis from the line of sight to a celestial body. i I g It is another object of this invention to provide ,a star sensing system adapted to yield an electrical signal which may be used to correct the orientation of an optical axis to coincide with the line of sight to a celestial body.

Another object of this invention is to provide a star sensing system in which the effect of sky gradient signals is substantially eliminated.

atent 2,941,081 Patented June 14, 1960 Other objects of invention will become apparent from the following description, taken in connection with the accompanying drawings, in .which Fig. 1 is a schematic elevational view of the invention;

Fig. 2 represents a plan view of reticles employed in this invention;

Fig. 3 is a block diagram of the electrical circuit of this invention;

Fig. 4 is a plan view of a somewhat modified form of the reticles shown in Fig. 2;

Fig. 5 is a detailed schematic of a computer of this invention; 1

Fig. 6 is a combined electrical diagram and end view of the device shown in Fig. 5; and i k Fig. 7 is a detailed view taken at line 77 in Fig. 5

Referring to Fig. 1, light from a star andfrom the sky immediately surrounding the star is gathered by a lens 1 and split by prisms 2 and 3 into two parallel light beams. These two beams fall on reticles 4 and 5 which are rotated by means of gears 6 and 7 in turn driven by motor 8 and gear 9. Reticles 4 and 5 are shown in plan view in Fig. 2. Gears 6 and 7 are of differing size, as is shown in Fig. 1, so that reticle 5 may be driven at two-thirds the speed of reticle 4. Light from reticles 4 and 5 falls upon photocells 10 and 11, respectively, which generate alternating current signals in response to the modulated light which falls upon them. A reference generator 12 is driven by motor 8 and furnishes a reference frequency S Althernating current signals S and S are generated by photocells 11 and 10, respectively. In Fig. 3 the signal from reference generator 12 is fed to amplifier 13 and thence to phase shifter 14 mechanically connected to computer 15. From phase shifter 14 a modified signal is fed to differential synchro 16. Photocell signals are fed to amplifiers 17 and 18 and from there-to differential synchro 19. A signal from amplifier 18 is fed to difierentialsynchro 16, as shown in Fig. 3.

In operation, light from the star is chopped by reticles 4 and 5 which are so constructed as to incorporate alternate transparent and opaque portions, the boundaries of which 'are Archimedes spirals having the formula R=k6, where R is the radius to any point on the curve from the center of the reticle; 0 is the angle between said radius and some reference axis; and k is a constant of proportionality,.hereinafter referred to as the spiral curvature of the spiral. Itwill be noted from Fig. 2 that reticle 4 comprises a two-lobed spiral, while reticle 5 has three lobes. Likewise, from Fig. 1 it will-be noted that .reticle 5 is driven at two-thirds the speedwith which reticle 4 is driven, with the result that the signals generated by both photocells have the same frequency. In the following analysis, k for the spiral of the three-lobed reticle will be arbitrarily-assigned the value of K, while 0 for the same reticle and a particular location of the star'in the field will be designated by The value of k for the two-lobed-spiral must be /a K.

Referring now .to Fig. 3, the signal output of photo-' cell 11 fed to amplifier 17 may be represented by The signal fed to amplifier 18 from photocell 10 may be represented by Y I I field, it being understood that the reticles spin counterclockwise'with respect-to'theseaxes; 1

These two signals are fed to differential synchro '19 whose shaft output, of cou'rsewhen properly indexed, i's'the phase difference betweens and S -,-or, sirnpl'ytb. It now becomes apparent "that unless'the spiral curvatures k for the spirals of-the tworeticles bear a ratio equal to the reciprocal of' the ratio ofxthe respective rotational, speeds of the reticles thcmiddle terms of the equations for S and S would not subtract out in the differential sync'hro. This sha'ft'output is connectedto phase shifter 14. .Phase shifter 14 also receives 'a ;sig-' nal S "from"reference"generato1f12 equal to sin 6rd. The output shaft of differential synchro' '19 is connected to'phase shifter 14 by gear box 20 having a'2:1"r'atio so that phase shifter 14 receives a rotation that shifts S "t5y"'an amount *2tl/i Asi'gnal-S :sin (6wt+2yb)'is therefore formed :and fed to differential synchro 116'.

distance andshaft '29 at 0, output gear '42 turns at vt), as

, 4 r Likewise, since shaft 24 turns at does shaft 43. Resolvers 44 and 45 are connected to shafts 43 and 41, respectively, for the purpose of producing a displacement proportional to the sines and cotween them; Resolv'er 44 comprises a wheel 46"to which Differential SYIlCh10 '16 also receives signal' S, so that the shaft ,output of differential synchro 16 "is "3 being the phase difference *between S and'iS Computerls therefore receives two shaft inputs proportional to 5 puter15 shown in "Fig. '6, which takes the sum and difference of these two rotations and-resolves them 'into sines and 'cosines; i.c., I v

.sin (ti-w), l -)Q in (uh-sh), c .(Wltl) Thereafter the differences give Sta (+).sin,( /4I)Z a4a t t =Vi 1 I P Wf Q f+?#%- t For small rotations sin-prapproximates up, it is .prace ticable :to use these signalsqto accomplish rections, respectively. 1 In general, N .and:2V need to be accurate only zneartthe center off-the fieldy'therefore, this: approximation is jnstified. w Referringz to Fig. 5, 'there.is: shown a schematic =view of computer .15. Shaft 11,-:driven1by differential .synchro 16 with an angularudisplacement '.'drives gear '22 which in rturn :drives'; gear #23 with .-.-an*'angular. l'displacement of -';Gear 23 is :lattached tot sh aft 24 which is integral with frame 25 having spur gearsi'26awand-ll7 freelyarotatable ithereon. iGea'r '28 meshing .with gears 26a. and :27 is freewtoirotatet on shaftr24; an'dz is attached by means TOf shaft 29.:to.gear130. .tGears 26a and 27' mesh with :gearstZGQ-and 27alalso carriedas spur gears on frameJZSJ: 1Gear'ss26: and 27a'd'rivewgea1: 42 attached to shaft .43. LGear' 53.0fmeshes :with idler gear 31 whi'ch in'zturn is drivenibyg1gearl32zattachcd to shaft =33 'which is 1 drivencby :phasel: shifter :14 t in Fig. 13, with Fan angular displacement of-s b. cGear' t23 fmesheswithi gear 34 attached to frame 35 having spur gearsv 36and 37a rotatable thereon, and gears 36a and'37a with gears V 36 and 37. -Gears"i36"and -37 irrturnrnesh with-'gcar 38 which is attached to shaft 39 rotatablez-infr'ame $5 and attached to gear 3 2 7 .Gear '38 tis ;ther. eforeidriven with an angular displacement of p; 7 Gear 40 is the out-. put'gear and is driven by 'gears '-'36 and f37' whi'ch, to-j ge'ther Withtheir associatedgears'ancl theframe, "con? stitute a spur :gear differential. ""kotationjof l'gear 40,

1,0, since-1pisfurnished asarotation tofthe frame the spur'gear' differential, p isj'furnished as there tatio'n -ofgear 38; "Shaft '41 "therefore "turns angular and x vcorisattached a,c rank g47lslidab1e in cross arms. :and 49 V which are irestrained :to move=onlyin the direction normal to the slots 50 and 51 of the cross arms. Oro'ss arms-48 and 49 are integrallyattached to racks 5,2,.and .53 which drive gears54 and '55, respectively;'"w'hich gears are mechanically connecte'dby'any suitableldevice to mechanical differentials 56 and 57. In a similar fashion, resolver comprises a Wheel 58 having a crank '59 movable in slots 60 and.6 1, of cross arms 62 and 63 restrained against 7 movement in' any'dircction other than normal ,to the longitudinaldir ection of slots 60, and61. Racks164 and 65 turn gears '66 and 6 7, r'espectiyely, which ,in turn are me-.

cha'nic'ally connected tomechanical differentials 56 ,'and' 57,. Mechanical differential 56'is mechanicallyconnected to rotate potentiometer 68, and mechanicalrdiiferential 57 is mechanically connected to rotate potentiometer v69'.

In operation, the outputfof shaft 41 is an angular distance proportional to (ml/4 While the angularrotation of shaft 43 is proportional to (\,/-V )..-f1"he rotationof gear 54 isthere'f'ore proportional to the sine of.(\//-); the rotation ofgear 55is proportionaltofthe cosine of (gb-qz); the rotation of gear ,56' isiproportional ,to the sine of and the rotation'of ,gear .67 is .proportional to the cosine ,of h t-dz). Gears 55 and'67 are connected as inputs to a.mechanica1 differential57 whose output turns potentiorneterffitl. Potentiometer 69 therefore has an angular rotation vproportional tocosine (\//q )''cosine (t//), which may betra'nsformedfl-in V accordance with a well-known trigonometricidentity to the expression 2 Sln d), shit/1; In a similar ma'nncr, gears 54 and tidare connectedrtoxhe' input of mechanical differ? ntial :56 whose output turns potentiometertfis. ,Potentior'rieter- 68 therefore experiences. an. angular rotation proformed -in accordance with ,a well-known ti"ig0 n0metrfic identity toL-sinEbQcosQP, l" a d L Qomputer.1 5,' it. will be appreciated,.mayl 7 replaced by ,v vellrknownfmechanical computer componentsfwhich may bevaried considerably in fo rrnto. achieve thezsarne result. For instance, resolvers 44 and A5 maybe replaced by such resolvers asthose shown in Figs. ,1,l5, and, 1.16

on, pages 17 and .18 of ComputingMechanisms,and Lir lcg Tby Antonin'Svob'oda. Similarly,. the spur-gear dif ferentials of which gears '26-,and36 area part may be replaced T by a spur ear; differential such ,as -,is shown in I These voltagesjrnaylthenflbe used asrcorrection 'voltages for changing the orientation of the telescope so that the optical: axis of 'theltelescope. coincides with'the' line of. sightito'thestar' being sighted; It will be noted that as these' corrections decrease in I magnitude, the Quantity 2 sintfi cos tlxapproaches'Krp cos 1,11, and the' quantity V 2sin' sin 1/ approaches K sinql/r Since, in. general, it is not required. to know the magnitucle of corr'ection required eitcept where the correctior'i becomesfsmall, the reticles 4a andI5ashown inFig. mayb'e used. 1 In 'these}reticles the spiral portion of the reticles occupies only the central portion thereof, and the remainder of the reticles'is merely sector-shaped ,Ihis

configuration results in a somewhat higher sensitivity and greater accuracy over a small central position of the field. It should be further pointed out that the spiral and radial sectors heretofore disclosed may be varied somewhat in shape so long as the centroid of the transparent areas of the reticle falls on the axis of rotation thereof. The spiral configuration is expedient because it simplifies the computer required to yield quantitative information for correction of the telescope orientation. So long as the cen troid of the transparent areas of the reticle falls on the axis of rotation the effects of sky illumination and the first order terms of sky gradient are eliminated. In other words, the modulation eifect of a uniform sky background light as well as that due to a linearly varying sky background light are both eliminated by virtue of the fact that the centroid of the area coincides with the axis of rotation of the reticle.

If E, represents a uniform sky illumination, the total illumination, neglecting higher than first order terms because of their negligible comparative size, may be represented by where E and E201 constants, and x and y are measured from the axis of rotation, as indicated in Fig. 2. The total flux to the photocell then would be where dA represents a diiferential'area of the field. Apparently, if f xdA and f ydA are Zero, F will be a constant. If the flux is constant, the photocell will generate no varying signal on account of the sky light or linear sky light gradient. But if these terms are zero, the centroid of the area is coincident with the axis of rotation of the reticle.

Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of this invention being limited only by the terms of the appended claims Weclaim:

1. In a star sensing system means for producing voltages indicative of the deviation of an optical axis from the line of sight to a star comprising an optical system for gathering light from a star, optical means for splitting saidlight into two beams, a first rotatable reticle having two symmetrically-disposed spiral boundaried transparent sectors in the path of one of said beams of light, a second rotatable'reticle having three symmetrically-disposed spiral-boundaried transparent sectors in the path of, the other of, said two beams of light, said last-named to receive light from said second reticle, difierential synchro means for subtracting the signals received from said two photoelectric cells, phase shifter means connected to the ouput of said differential synchro means and to the output of said generator means, a second differential synchro connected to the outputs of said second photoelectric cell and said phase shifter means, and computer means connected to the output of said phase shifter means and to the output of said differential synchro means for producing voltages indicative of the magnitude and direction of the deviation of a line of sight to said star. from the optical axis of said optical means.

2. Means for detecting the deviation of a line of sight to a star from an optical axis comprising an optical system for gathering light from said star, optical means for splitting said light into two similar beams, a first rotated reticle having two symmetrically-disposed sector-shaped transparent areas whose centroid coincides with the axis of rotation thereof, a second rotating reticle in the other of said light beams having three symmetrically-disposed sector-shaped transparent areasiwhose centroid coincides with the axis of rotation thereof rotated at two-thirds the speed of rotation of said first rotated reticle, said reticles being characterized by having their transparent areas bounded by Archimedes spirals in at least their central portion, the spiral of said first-named reticle having a spiral curvature two-thirds that of said second reticle, a photoelectric cell for producing an alternating current signal in response to the modulated light falling upon it from said first reticle, a second photoelectric cell for producing an alternating current signal in response to modulated light falling upon it from said second rotated reticle, reference generator means for producing an electrical alternating current signal proportional in frequency to the speed of rotation of said reticles, and computer means responsive to said alternating current signals for producing voltages proportional to the coordinates of displacement of the image of said star in the field of said optical system. i

3. Star sensing means comprising an optical system having a field and an optical axis for gathering light from a star and splitting it into two simliar beams, means for transmitting unmodulated the background light from the sky adjacent said star and for modulating the starlight in said two beams at different frequencies and with a phase relation dependent upon both the magnitude and sense of the displacement of the image of said star from the center of said field, two photocells for generating electrical signals in response to said beams of light, and means responsive to said photocell signals for generating voltages whose phase is proportional to the deviation of said optical axis from a line of sight to said star.

,4. A device as recited in claim 3 in which said modulating means comprise two rotating reticles, the centroid of whose transparent areas falls at the axis of rotation thereof; and means for rotating said reticles at predetermined constant speed to thereby eliminate the eflfect of sky gradient. l

5. A device as recited in claim 3 in which said modulating means comprise two rotating reticles having trans,- parent areas the combined centroids of which coincide wtih their axes of rotation and whose transparent areas are bounded by Archimedes spirals to thereby modulate the light from said star with the phase relation depending upon the displacement of the image of said star from the center of said field.

6. A device as recited in claim 3 in which said modulating means comprise two rotating reticles having transparent areas bounded by Archimedes spirals whose centroid coincides with the axis of rotation of said reticles and in which said reticles are rotated at predetermined differing constant speeds to thereby modulate the light in said two beams with -a phase relation dependent upon the magnitude and sense of the displacement of the image of said star from the center of said field.

7. Star sensing means comprising an optical system including optical means for splitting the light from a star into two similar beams, and a telescope having a field and an optical axis, a rotating reticle in each said beam of light, one of said reticles having two transparent areas bounded by Archimedes spirals the centroid of said areas being coincident with the axis of rotation of said reticle, a second reticle in the other of said beams having three transparent areas bounded by Archimedes spirals the spiral curvature of which differs from that of the Archimedes spirals of said first reticle, and the centroid of whose transparent areas coincides with the axis of rotation of said second reticle; means for rotating said reticles at predetremined constant differing speeds; and photocell means for producing electrical signals in response to the modulated light from each said beam of light whereby 7 V the phase relation of the light modulated' by said reticles is indicative of the departure of 'saidjstarfimage from the center of said field.-

'8. Star sensing means comprising an ioptical system including light beam splitting means for splitting the light from a star into two similar beams and a'telescope having a field and an optical axis,-'rotatiug reticle-meansfor modulating the light in'each of said'beams;{each jsaid rotatable reticle having transparent areas whose centroid coincides with the axis of rotation of said 'reticleand whose boundaries are"Arehimedesspiralsof predetermined differing spiral curvatures in each' said reticleymeans for rotating said reticles at predetermined difiering 'con stantspeed s, photocell means' for generating electricalsignals in response to [said modulated li'ght, and ele'ctrical means for generating voltages 'i-ndicative' of thedeviat-ion' of a line of sig ht'tosaid star from saidoptic'al axis in re sponse to said photocell signalswhereby saidoptic'a'l axes may lie/trained onsaid star. 1 a a 1 p r I 9. In a star sensing device having 'a telescopefor gathering light from a star, the combinationof optical means for splitting the light gathered by saidtel'escop'e into two similar beams, two photoelectric cells for generating electrical' signals in response 'to the light in each of said beams,.a rotatable reticle in one of said beams-having two symmetrically-disposed congruent transparent areas whose boundaries are SimiIarArchimedes:spirals, a second'rotatable reticle having three syriunetrically-disposrd congruent transparent areas whose boundaries are Achi medes spirals having spiral curvatures of'predeterminately difiering value from those of said first reticle'and intersecting at the axis of rotation of said reticle," means for rotating said reticles at constant but predeterminatel-y differing speeds, an alternating current generator adapted to yield a signal whose frequency corresponds 'to the speed of rotation of said reticles, and computer means for pro ducing voltages indicative of thedeviation or the line of sight-to a star from the'optical axis of said telescope in response to the signals from said photoelectric cells and said generator. Y i r 7 10. :In a star -s ensing.dev ice having a telescope,=means for modulating the light from a star in a manner 'indic-- ativeof -the magnitude and direction: of deviationcf-the line of sight to said star from the optical aX'is'of said telescope comprising optical means-ion splittirigthe light from a star into two similar beams, -a first rotatable reticle having two congruent;'tran'sparentareasbounded by Archimedes spirals the centroid of said transparent {areas being coincident with the axis of rotationof sa'id reticle, a second rotatable reticle having three congruent tr-ans parent areas hounded by Archimedes spirals" of prede terminately differing "spiral curvature from the spiralsin said first reticle, ,the centroid of said transparent areas being coincident with the axis of rotation oi said reticle,- and means for rotatingsaid reticles at constant predeterminedarelative speeds to thereby modulate the light from said star'lso as .to indicatethe magnitude and direction of deviation of a :line of sight to said star 'from'the optical axis of said telescope V g 7 11. In a star sensing device having anoptical system which has a field and an optical axis for gathering light from a star and splittingit into two similar 'be'amsyrneansfor modulating the light in each of "said'two' beams corn prising two rotating reticles each having a plurality of transparent areas bounded by curves whose radius vectors vary with the angular displacement of? a point-thereon"in accordance with a predeterminedqfunction and lwliosecem troidfalls at theaxis-of rotation thereof, therebyeliminating the effect of sky intensity-gradient; meansforrotati'ngsaid reticles at predetermineddifferent constant speeds;

two photocell s for, generatingsignals in response to said' beams of light; and, electronic means-responsive to said photocell signals for generating voltages proportional to thedeviation of said optical axis fromaline of sight'tQ said star."

' ent areas 'the combined-centroids of-which coincide with i 8 1 2. In star sensing device having an optical system with a tieldjand-a'noptical-axis for gathering" lightfrom astar and splittin'g it into two similar beams; means for modulating the light in said twdbe'ams a't 'difierent freqnen-cies comprising two rotating reticles havin'g-transpar the' axes of rotation 0t said reticles Sand whose transparent areas are bounded by Archimedes spirals; means for rotating said reticle's at predetermined constant speed;

two .p'hotocells for-generating electrical signals in -response torsaid beams of light "wit-ha phase relation dee pendent upoir the .magnitude and sense offv the displacement oflhe'irna'ge of said star from thecenterofsaid -fi'eld; and-i electronicmeans responsive .to .saidrphotocell signals 'for generating voltages proportionalto the ldevi ationiof said optical from a-line of sight :to vsaid star.

a 13. :hr a-"star sensing device having an optical system withiaa-field and anfoptical'axis for 'gatheringlight fitom'. a;starrand splittingit into two similar beams; means for modulating the light comprising two rotating reticl'es hav-; ing transparent areas "bounded byf Archimedes spirals whose centroid coincides with the axis of rotation ofsaid V reticles and in which said reticles are rotated at predeter rnined differing constant speeds to thereby f'medulate the v light in said two beams with a' phase relation dependent upon the magnitudeand sense 'ofthe' displacement of the image of said star from the center of saidvfield.

14. lnla radiant energy sensing device having an opti tical. system for gathering light from a radianfenergy source theicombination of opt ical means for gathering" at the axis of rotationof said, reticles having spiral eurva 8 of predeterminately differing value fromfltliose of saidfirst'reticlq means for rotating said reticles at, con:

oles, and computennieans' for stant biitpredeterminately diiferent speeds jan alternatingfcur'rent'generator adapted toyield asi'gnalwhose fi e-f queasy corresponds tothe speed of rotation of said ret' five of the deviationloff theljline of sight to. said radiant energy source Iro'm'the optical axis of said 'optical'sysj:

detec n; device'and sa'id, generator.

tern infr'esponse' 'toithesig'nalsjfrom said radiant energyi 1' In 'a radiantenergysensing devieehavingtaniopti f' eaisy tem, means for ino'dulating the radiant energy from: a radiant energy source 'in' a manner indicative of the magnitude and'direction of deviation of the line of sight tosaid radiant energy ,source from thebpfical axis'nof 7 said optical system comprising optical means for. splitting, the r'adiant energy from a radiant energy source into two similar beams, a first rotatable reticle having two congnlent'transparent areas bounded by Archimedes spirals the centroid of said transparent areasbeingv coin cide'nt withthe axis of rotation of said reticle, a secondi rotatable reticle having threecongruent transparent' area s, hounded by Archimedes spirals of predeterminately differ-1 ing' 'spiralcurvatur'e fromthe spirals in said "first reticle,

the, centroid of said transparent areas being coincident with-the axis of rotation of said reticle, and means for otating s'aidreticles at constant predetermined relative speeds to thereby modulate the radiant energy'fromsaid radiant e'nergy sourcefsoas to indicate the magnitude and direction of dev'iation of a lineof sight to said Fradiant L}l6. a star sensingfde vice --having a telescope for gathering v light from a star, the .combinationof optical energy source from the optical axis of said optical sys-' oducing 'voltages fiiidi'ca-;

9 z'rheans for splitting the light gathered by said telescope into a plurality of similar beams, a plurality of photoelectric cells for generating electrical signals in response to the light in each of said beams, a rotatable reticle in each of said beams, each of said reticles being provided with a different plural number of symmetrically disposed congruent transparent areas whose boundaries in each of said reticles are similar Archimedes spirals intersecting at the axis of rotation thereof, the spiral curvature of the spirals on each of said reticles being of predeterminately difierent value from those of any other of said reticles and the centroid of the transparent areas of each of said reticles coinciding with the axis of rotation of said reticle, means for rotating said reticles at constant but predeterminately different speeds, an alternating current generator adapted to yield a signal whose frequency corresponds to the speed of rotation of said reticles, and computer means for producing voltages indicative of the deviation of the line of sight to a star from the optical axis of said telescope in response to the signals from said photoelectric cells and said generator.

17. In a star sensing system, a rotating reticle having a plurality of symmetrically disposed congruent transparent areas whose centroid is at the axis of rotation of said reticle, the boundaries of said transparent areas being similar Archimedes spirals 'whose spiral curvature has a predetermined value.

References Cited in the file of this patent UNITED STATES PATENTS 

